1. Field of the Invention
The present invention relates generally to techniques for reducing the effects of jamming in radio-frequency receivers and more specifically to methods and apparatus for enhancing the reception of global positioning system (GPS) signals in the presence of jamming signals.
2. Description of the Related Art
Each satellite in the GPS transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHz and an L2 signal at a frequency of 1227.6 MHz. The L1 and L2 signals are bi-phase signals modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. The GPS signals are subject to disruption by jamming signals, which may be transmitted either as narrow band signals or broadband signals. U.S. military forces utilize both L1 and L2 which are both subject to jamming interference.
U.S. Pat. No. 5,952,968, issued to present applicant C. E. McDowell, entitled, “Method and Apparatus For Reducing Jamming By Beam Forming Using Navigational Data,” discloses a method of reducing jamming in a global positioning system (GPS) satellite receiving system that includes the steps of: a) selecting an initial weight value corresponding to each antenna element; b) selecting a weight adjustment scheme for adjusting the weights; c) measuring a power output from the antenna array; d) obtaining navigational data representing the orientation of the array; e) calculating for a gain of the antenna array corresponding each of the GPS satellites, using the navigational data to provide an indication of the orientation of the array relative to each GPS satellite; f) estimating a power level of a received signal corresponding to each GPS satellite; g) solving for the signal to noise ratio for each GPS satellite, using the estimated power level corresponding to each GPS satellite; h) iteratively and continuously adjusting the weights to obtain a greatest value of the signal to noise ratio; and i) continuously repeating steps (c) through (h).
U.S. Pat. No. 6,933,885, issued to Stockmaster, et al., entitled, “Miniaturized Digital GPS Anti-jam for Space and Size Constrained Applications,” discloses a system for providing anti-jam protection that includes a two element antenna; a radio frequency downconverter providing an in phase and quadrature component of received signals by the two element antenna; and at least two analog to digital converters. The analog to digital converters are coupled to the radio frequency downconverter, wherein a first channel complex data and a second channel complex data formed from an output of each of the at least two analog to digital converters form a covariance matrix. A weight calculation is obtained from the covariance matrix providing optimal anti-jam suppression. The calculated weight for optimal anti-jam suppression is applied through hardware components.
U.S. Pat. No. 6,421,000, issued to present applicant C. E. McDowell, entitled “GPS Multipath Mitigation Using a Multi-Element Antenna Array,” discloses a method and apparatus for mitigating multipath signal distortion using a multi-element antenna array. The multi-element antenna array is used to discriminate between the desired signal and its multipath components based on spatial angle of arrival. A reference signal, provided by the positioning system receiver, is used to compute element weightings that are utilized to null out the multipath components before they reach the receiver.
U.S. Pat. No. 5,990,831, issued to present applicant C. E. McDowell, entitled, “FFT Implementation of Digital Antenna Array Processing In GNSS Receivers,” discloses global navigation satellite system (GNSS) receivers, including digital spatial nulling arrays, and a method of providing antenna pattern outputs using the same. The digital spatial nulling array receives multiple antenna element inputs. A fast Fourier transform (FFT) is applied to the multiple antenna element inputs to obtain frequency domain representations of the multiple antenna element inputs. The antenna pattern outputs are provided by the spatial nulling array as a function of the frequency domain representations of the multiple antenna element inputs.
Referring now to FIG. 1 (Prior Art) the architecture of a normal L1 Only Global Positioning System (GPS) receiver is illustrated, designated generally as 10. The term “normal L1 Only Global Positioning System (GPS) receiver” refers to a GPS receiver that can only receive an L1 band satellite GPS signal. The receiver 10 may include an antenna 12, an L1 band pass filter (BPF) 14, an L1 RF downconverter 16, an analog-to-digital converter (ADC) 18 and a baseband GPS processor 19.
Referring to FIG. 2 (Prior Art), the architecture of a normal simultaneous L1/L2 GPS receiver is illustrated, designated generally as 20. An L1/L2 diplexer 22 is used to receive GPS signals from antenna and then feed L1 and L2 band signals to an L1 RF downconverter 23 and L2 RF downconverter 24 in a parallel manner. After being converted by respective ADC's 26, 27, signals from both the L1 band and L2 band will be processed by a baseband GPS processor 28. This receiver can receive GPS signals from both L1 and L2 bands, but it is more expensive than the normal L1 only GPS receiver 10.
Referring to FIG. 3 (Prior Art), an integrated anti-jamming GPS receiver 30 is illustrated. This includes L1/L2 Diplexer 31, a switchable L1/L2 RF downconverter 32, an ADC 33, digital anti-jamming (AJ) processor 34 and baseband GPS processor 36. Anti-jamming is made possible by the digital anti-jamming processing before GPS signals are processed by the baseband GPS processor.
A federated L1/L2 AJ system would use a switchable L1/L2 RF downconverter that downconverts an L1 or L2 RF signal to IF or near-baseband as required for A/D conversion and AJ processing. Then it would upconvert the signals after AJ processing to L1 or L2 band RF signals according to the source before AJ processing. However, this would result in relatively big and expensive AJ GPS systems.